Expansion tank and regulator back flow pressure issue

This past winter I replaced my HWH and Expansion tank (both were bad). However when the HWH heated up to the Medium set point the relief valve started spitting water. Basically it was doing its job. I went over to the Regulator back flow valve and turned it down a lot. That seemed to stop the HWH from spitting. But now that summer is here the water pressure doesn't even work the lawn sprinklers properly. I have roughly 42 psi at the hose bib. It was around the same when I tested the HWH with the hot water running out of a different sink. The new Expansion tank was factory set at 40 psi with a max of 150. My HWH is a 50gallon capacity and the regulator back flow valve can range from 25-75 depending on set point.

My question is this. If I were to boost the regulator up to 60 psi would I also need to set the Expansion tank at 60 psi? Do they need to be equal or does one need to be greater than the other?

*sigh* I was all over the Watts website looking for something like your link and couldn't find it yesterday. Thanks for responding.
Have an awesome 4th of July and don't forget to say thanks to a veteran.
Peace,
Chuck

BTW...I did check the pressure which was at 40. I bumped that up to 60 at the regulator, drained the lines, and set the exp tank to 60. All is working great. I can actually use the sprinklers now.
Thank you for serving this great country!
May God bless you and all our troops.
Again, Happy 4th
Chuck

Actually to get full use of the capacity of the tank, the air charge is supposed to be set 2 PSI below normal static water pressure, with no water pressure pushing against it. But the same is good enough.

The two times the air charge is critical is if it is ZERO, meaning there is no air volume to absorb expansion, or higher than the tank's T&P valve's setting, i.e., 150 psi, so there is no way the water pressure could overcome it to enter the tank.

Actually to get full use of the capacity of the tank, the air charge is supposed to be set 2 PSI below normal static water pressure, with no water pressure pushing against it. But the same is good enough.

Click to expand...

That would be the recommendation for well tanks...
For expansion tanks this is not the case and as HJ stated pressure is not critical unless it is way way off...

Per Watts:
The expansion tank pre-charge must be set to the system pressure as
determined in Step 1. Pre-charge prior to installation in the system.
Caution: Pre-charge prior to installation in the system.
Do not adjust the air pre-charge of the expansion tank
with the system under pressure. The air pre-charge
should only be adjusted under zero system pressure.

My recommendation for 2 PSI below works best as to use full capacity of tank as I stated earlier

Difficult to argue, following instructions, and not making up your own theories makes for an easier and proper install with less callbacks.

Let me remind you, master plumber title does not make you manufacturer product experts.